Organic phosphites are known in the art as secondary antioxidants for polymeric resins such as polyolefins and elastomers. As an antioxidant, these phosphites are oxidized to phosphates to prevent oxidation of the polymer. Examples of such phosphites are disclosed in H. Zweifel (Ed) Plastics Additives Handbook, 5th edition, Hanser Publishers, Munich 2000. One common problem for most phosphites is the tendency to undergo unfavorable hydrolysis upon exposure to moisture or water, even trace amounts, during storage or handling. Initially, hydrolysis of the phosphite generates acidic P—OH and PH═O protons that are good reducing agents that react directly with oxygen or hydroperoxides. However, if hydrolysis continues past this initial stage, stronger acids are formed that greatly accelerate the formation of oxidized products. Additionally, other acids from impurities arising from residues of polymerization catalysts may further catalyze the phosphite hydrolysis. These oxidized products lessen the overall ability of the phosphite stabilizer to function as an antioxidant. As a result of exposure to water, hydrolyzed phosphites become a lumpy, sticky mass that leads to corrosion of processing equipment.
Conventionally, to prevent hydrolysis, producers have sought phosphites that are slow to hydrolyze and have added various hydrolysis stabilizers to the phosphites. U.S. Pat. No. 3,787,537 describes a triisopropyl phenyl phosphite ester that is slow to hydrolyze in combination with a heavy amine to further increase the stability to hydrolysis.
Trialkylaryl phosphite stabilizers having hindered alkyl groups at the ortho and para positions are resistant to hydrolysis due to steric hindrance. One of the most widely used phosphites is tris(2,4-di-t-butylphenyl)phosphite, which is commercially sold under the trade name Alkanox™ 240, Irgafos™ 168 or Doverphos™ S-480. This phosphite is a solid and is commercially available without a hydrolysis stabilizer.
Other trialkylaryl phosphite stabilizers, such as the widely used tris(p-nonylphenyl) phosphite (TNPP) are susceptible to hydrolysis. TNPP is a liquid at room temperature. Commercial grades of TNPP, such as Weston™ 399 (Chemtura Corporation), usually contain up to 1 wt % of triethanolamine or triisopropanolamine, which acts as an hydrolysis stabilizer.
U.S. Pat. No. 5,561,181 discloses a highly ortho-substituted TNPP that is more hydrolytically stable than para-substituted TNPP.
EP0167969 discloses a phosphite that is hydrolytically stabilized with a long-chain aliphatic amine, such as coconut-alkyl diethanolamine. EP0143464 discloses a pentaerythritol diphosphite that is hydrolytically stabilized with a long-chain aliphatic amine, such as octyldecyl diethanolamine.
There is, however, a need to replace TNPP owing to alleged estrogenicity concerns associated with nonylphenol, which is used in synthesizing TNPP.
Thus, the need exists for safe and effective liquid phosphite compositions for use as secondary antioxidants in polymers that may be hydrolytically stabilized.
In addition, there is a need for amine compounds that are suitable for the hydrolytic stabilization of a wider range of phosphite antioxidants.